This invention relates to microfilm having improved properties. In one of its more particular aspects this invention relates to microfilm in which images contained on the microfilm are less susceptible to fading upon exposure to light than ordinary microfilm.
Diazo microfilm is typically made by applying a coating of, for example, 0.1 mil (2.5 microns) to 0.2 mil (5 microns) of a cellulose alkanoate to a polyester film base. This coating may incorporate light sensitive diazo chemicals or they may be impregnated into the layer from solvent solutions in a separate step. Such a product is then developed by application of an alkaline developer. Although microfilm prepared in accordance with this method is generally satisfactory, if exposed to radiation such as strong light in microfilm readers for extended periods of time, fading may occur with the result that in time the microfilm images may become illegible. For instance, as much as 90% of the image may be lost in a period of 16 hours exposure to the light from a microfilm reader.
Surface saponification of cellulose alkanoate films has been previously disclosed for the purpose of enhancing the substantivity of such films to photosensitive solutions. Dinaburg, for example teaches that the problems caused by non-uniform distribution of the photosensitive solution on the surface of the base layer of the film can be eliminated by coating the photosensitive solution onto the surface of cellulose which has previously been made hydrophilic by hydrolysis to a depth of 3-4 microns. (Dinaburg, M.S., Photosensitive Diazo Compounds and Their Uses, The Focal Press, London and New York). Other references teach superficial hydrolysis or deacylation, usually to a depth of approximately 4 microns. U.S. Pat. Nos. 1,340,416; 2,238,044; 3,311,475; 3,338,713; 3,397,058; 3,397,985; 3,442,651; and 3,442,652 contain such teachings. Dutch Patent application 72/07099 teaches surface hydrolysis to a depth of 5 microns. U.S. Pat. No. 3,365,293 teaches hydrolyzing a cellulose alkanoate to a depth of 5-10 microns.
None of the prior art, however, discloses controlled hydrolysis of a cellulose alkanoate layer to an extent defined by certain infrared absorption parameters to produce a fade-proof microfilm.